The present invention relates, in general, to a method and apparatus for controlling the direction of drilling a borehole. More particularly, the invention is directed to a method and apparatus for controlling the bending of a rotary drill stem with respect to the borehole to control the direction of drilling, and still more particularly to a multiple cam actuator for producing an accurate and controlled bending of a rotary drill stem.
One commonly-used method for controlling the direction of drilling a borehole is to utilize a "bent sub" at the bottom end of a drill stem, with a hydraulically-driven motor being mounted on the bent sub for operating the drill head. The bent sub positions the axis of the drill bit at a slight angle with respect to the axis of the drill stem and the hydraulic motor drives the drill bit at the angle of the bent sub, for example, at an angle of about one degree from the axis of the drill stem. The drill advances in the direction of the bend, thereby causing the borehole to curve in the direction of the bent sub and the angular position of the drill stem controls the angular direction of the curve. To drill a straight hole, the drill stem is continuously rotated while the drill bit is driven to thereby rotate the direction of the bent sub around the axis of the drill stem. This makes a slightly larger borehole, but causes it to be drilled in a straight line.
Alternatives to the foregoing technique for directional drilling include the technique described in British Patent 2,177,738, published Aug. 3, 1988. This patent discloses a steerable rotary drilling technique wherein a rotary drill stem passes through an enclosure tube, or housing, which is held against the sidewall of the borehole being drilled so that the tube does not rotate. Inside the tube, a system of hydraulically inflatable bags deflect, and thus bend, the drill stem in a controlled way with respect to the enclosure tube, and this bend causes the axis of the drill bit outside the housing to be angled with respect to the axis of the tube and thus of the borehole, causing the drill to advance in the direction of the bend to produce a curved borehole.
Another control mechanism for steerable rotary drilling systems is described in a publication of J. D. Barr et al entitled "Steerable Rotary Drilling with an Experimental System" presented at the 1995 SPE/IADC drilling conference held Feb. 28, 1995 (Paper Number SPE/IADC 29382). As there disclosed, a control mechanism selectively deflects drilling fluid against one of three radial pistons which extend out of the drill stem and against the wall of the borehole. The pistons are sequentially pushed outwardly as the drill stem rotates to press the drill stem away from a selected point on the borehole sidewall to thereby apply lateral force for steering the direction of drilling.
A third known control mechanism is illustrated in U.S. Pat. No. 5,168,941, wherein a stabilizer is anchored to the borehole wall. Drilling fluid actuates four pistons which press against the wall to adjust the location of the stem and to thereby apply lateral force to the drill bit to cause the borehole to curve.
Other control devices have been developed in the art to control drilling direction though the use of eccentric cams which bend the drill stem, as described, for example, in U.S. Pat. Nos. 5,307,885 and 5,316,090.
Each of the foregoing systems has been found to have problems, not only in producing an accurately controllable deflection in the drill stem, but in measuring the actual deflection produced by the control mechanism. Accordingly, there is a need for an improved control mechanism for directional drilling utilizing rotary drill stems.